An emulsion is a fluidic state which exists when a first fluid is dispersed in a second fluid that is typically immiscible with the first fluid. Examples of common emulsions are oil-in-water and water-in-oil emulsions. Multiple emulsions are emulsions that are formed with more than two fluids, or two or more fluids arranged in a more complex manner than a typical two-fluid emulsion. For example, a multiple emulsion may be oil-in-water-in-oil (“o/w/o”), or water-in-oil-in-water (“w/o/w”). Multiple emulsions are of particular interest because of current and potential applications in fields such as pharmaceutical delivery, paints, inks and coatings, food and beverage, chemical separations, and health and beauty aids.
Typically, multiple emulsions of a droplet inside another droplet are made using a two-stage emulsification technique, such as by applying shear forces or emulsification through mixing to reduce the size of droplets formed during the emulsification process. Other methods such as membrane emulsification techniques using, for example, a porous glass membrane, have also been used to produce water-in-oil-in-water emulsions. Microfluidic techniques have also been used to produce droplets inside of droplets using a procedure including two or more steps. For example, see International Patent Application No. PCT/US2004/010903, filed Apr. 9, 2004, entitled “Formation and Control of Fluidic Species,” by Link, et al., published as WO 2004/091763 on Oct. 28, 2004; or International Patent Application No. PCT/US03/20542, filed Jun. 30, 2003, entitled “Method and Apparatus for Fluid Dispersion,” by Stone, et al., published as WO 2004/002627 on Jan. 8, 2004, each of which is incorporated herein by reference.